In order to clean and partially descale metallic strip, an operation essential with steel strip before finish rolling, it is standard practice to pass the strip through a succession of acid baths. The pickling apparatus therefore normally has upstream and downstream bridle roller pairs that flank an elongated housing and that pull the strip through this housing. Internal weirs, that is walls extending up from the floor of the housing, subdivide this housing into a succession of vessels each of which holds a respective acid bath, normally of Fe--HCl or H.sub.2 SO.sub.4 and of different concentrations.
The workpiece rests on the upper edges of the weirs and hangs by its own weight between these upper edges to droop below the liquid level of the bath and thus pass through the bath. The treatment time in each bath is therefore dependent on the bath length, transport speed for the strip, and amount of droop of the strip between adjacent weirs. The droop is in turn a function of strip thickness, bendability, and tension.
It is quite difficult to get good results with this type of setup. If the strip is too stiff it will not hang down enough to enter the baths appreciably, so transport speed must be slowed enormously to compensate by increasing residence time in the bath. Similarly if the strip is too limp it will be etched excessively unless it is moves very rapidly or is tensioned considerably. If the strip hangs too loosely in the baths it can rub the walls or floor of the apparatus and wear these parts out. Since band tension will have to vary between about 3000 N and 60,000 N for the necessary adjustment, a 20:1 variation, it is necessary to provide complex tensioning equipment.
In order to achieve uniform treatment it is possible to provide in each bath upstream and downstream upper rollers above the liquid level and upstream and downstream lower rollers below the liquid level and between the upper rollers. The strip passes over the upstream upper roller, then down and under the two lower rollers, then up and over the downstream upper roller. Such an arrangement, however, is extremely troublesome to operate. The rollers are quickly damaged by the acid and worn by the strip deflected around it. In addition the strip is subjected to so-called semiplastic deformation which can degrade it. Finally considerable energy is lost to the deformation and wear caused by the back-and-forth passage of the strip through the apparatus.
It is known to provide the apparatus with upper partition portions which form with the tops of the respective weirs slots some 100 mm-150 mm high, that is parallel to workpiece thickness, through which the strip passes with considerable clearance. These slots can be coarsely adjusted to prevent splashing of the one bath into the next one. Nonetheless this arrangement still allows the strip to carry considerable liquid from one bath to the next downstream bath, so that any accurate chemical balance established in these baths is quickly destroyed.